E.1. Restrictions on Stored Programs

Some of the restrictions noted here apply to all stored routines; that is, both to stored procedures and stored functions. Some of these restrictions apply to stored functions but not to stored procedures.

The restrictions for stored functions also apply to triggers.

Stored routines cannot contain arbitrary SQL statements. The following statements are not permitted:

The table-maintenance statements CHECK TABLE and OPTIMIZE TABLE. This restriction is lifted beginning with MySQL 5.0.17.
The locking statements LOCK TABLES and UNLOCK TABLES.
ALTER VIEW. (Before MySQL 5.0.46, this restriction is enforced only for stored functions.)
LOAD DATA and LOAD TABLE.
SQL prepared statements (PREPARE, EXECUTE, DEALLOCATE PREPARE). Implication: You cannot use dynamic SQL within stored routines (where you construct dynamically statements as strings and then execute them). This restriction is lifted as of MySQL 5.0.13 for stored procedures; it still applies to stored functions and triggers.
Generally, statements not permitted in SQL prepared statements are also not permitted in stored programs. For a list of statements supported as prepared statements, see Section 13.5, “SQL Syntax for Prepared Statements”.
Because local variables are in scope only during stored program execution, references to them are not permitted in prepared statements created within a stored program. Prepared statement scope is the current session, not the stored program, so the statement could be executed after the program ends, at which point the variables would no longer be in scope. For example, SELECT ... INTO local_var cannot be used as a prepared statement. This restriction also applies to stored procedure and function parameters. See Section 13.5.1, “PREPARE Syntax”.
Inserts cannot be delayed. INSERT DELAYED syntax is accepted but the statement is handled as a normal INSERT.
Within stored programs (stored procedures and functions, and triggers), the parser treats BEGIN [WORK] as the beginning of a BEGIN ... END block. Begin a transaction in this context with START TRANSACTION instead.

For stored functions (but not stored procedures), the following additional statements or operations are not permitted:

Statements that perform explicit or implicit commit or rollback. Support for these statements is not required by the SQL standard, which states that each DBMS vendor may decide whether to permit them.
Statements that return a result set. This includes SELECT statements that do not have an INTO var_list clause and other statements such as SHOW, EXPLAIN, and CHECK TABLE. A function can process a result set either with SELECT ... INTO var_list or by using a cursor and FETCH statements. See Section 13.2.8.1, “SELECT ... INTO Syntax”, and Section 13.6.6, “Cursors”.
FLUSH statements.
Before MySQL 5.0.10, stored functions created with CREATE FUNCTION must not contain references to tables, with limited exceptions. They may include some SET statements that contain table references, for example SET a:= (SELECT MAX(id) FROM t), and SELECT statements that fetch values directly into variables, for example SELECT i INTO var1 FROM t.
Stored functions cannot be used recursively.
Within a stored function or trigger, it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.
If you refer to a temporary table multiple times in a stored function under different aliases, a Can't reopen table: 'tbl_name' error occurs, even if the references occur in different statements within the function.
A stored function acquires table locks before executing, to avoid inconsistency in the binary log due to mismatch of the order in which statements execute and when they appear in the log. Statements that invoke a function are recorded rather than the statements executed within the function. Consequently, stored functions that update the same underlying tables do not execute in parallel. In contrast, stored procedures do not acquire table-level locks. All statements executed within stored procedures are written to the binary log. See Section 18.6, “Binary Logging of Stored Programs”.

Although some restrictions normally apply to stored functions and triggers but not to stored procedures, those restrictions do apply to stored procedures if they are invoked from within a stored function or trigger. For example, if you use FLUSH in a stored procedure, that stored procedure cannot be called from a stored function or trigger.

It is possible for the same identifier to be used for a routine parameter, a local variable, and a table column. Also, the same local variable name can be used in nested blocks. For example:

CREATE PROCEDURE p (i INT)
BEGIN
  DECLARE i INT DEFAULT 0;
  SELECT i FROM t;
  BEGIN
    DECLARE i INT DEFAULT 1;
    SELECT i FROM t;
  END;
END;

In such cases the identifier is ambiguous and the following precedence rules apply:

A local variable takes precedence over a routine parameter or table column
A routine parameter takes precedence over a table column
A local variable in an inner block takes precedence over a local variable in an outer block

The behavior that variables take precedence over table columns is nonstandard.

Use of stored routines can cause replication problems. This issue is discussed further in Section 18.6, “Binary Logging of Stored Programs”.

INFORMATION_SCHEMA does not have a PARAMETERS table until MySQL 5.5, so applications that need to acquire routine parameter information at runtime must use workarounds such as parsing the output of SHOW CREATE statements or the param_list column of the mysql.proc table. param_list contents can be processed from within a stored routine, unlike the output from SHOW.

The --replicate-wild-do-table=db_name.tbl_name option applies to tables, views, and triggers. It does not apply to stored procedures and functions, or events. To filter statements operating on the latter objects, use one or more of the --replicate-*-db options.

There are no stored routine debugging facilities.

Before MySQL 5.0.17, CALL statements cannot be prepared. This true both for server-side prepared statements and for SQL prepared statements.

MySQL does not support UNDO handlers.

MySQL does not support FOR loops.

To prevent problems of interaction between server threads, when a client issues a statement, the server uses a snapshot of routines and triggers available for execution of the statement. That is, the server calculates a list of procedures, functions, and triggers that may be used during execution of the statement, loads them, and then proceeds to execute the statement. This means that while the statement executes, it will not see changes to routines performed by other threads.

For triggers, the following additional restrictions apply:

Triggers currently are not activated by foreign key actions.
The RETURN statement is not permitted in triggers, which cannot return a value. To exit a trigger immediately, use the LEAVE statement.
Triggers are not permitted on tables in the mysql database.
The trigger cache does not detect when metadata of the underlying objects has changed. If a trigger uses a table and the table has changed since the trigger was loaded into the cache, the trigger operates using the outdated metadata.

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